There is considerable interest in photo-induced second harmonic generation for use in light sources for optical data storage. The phenomenon of second harmonic generation, which essentially doubles the frequency of an input beam, is useful in a frequency doubled light source. Of particular interest are second harmonic generators which can convert the infrared outputs of semiconductor lasers into short wavelength visible light in the green and blue portion of the spectrum. Higher frequency light not only can carry more information but also can be used to store and read information in correspondingly higher densities.
Photo-induced second harmonic generation has been observed in glass doped with germanium. The phenomenon has generally been attributed to defects produced by the germanium dopants. The reproducible efficiencies have been less than about 10.sup.-3 % per watt and are generally considered too low for use in practical devices.